Field
This disclosure relates generally to semiconductor devices, and more specifically, to temperature calibration for semiconductor sensor systems.
Related Art
Many systems utilize sensors to monitor and/or control the operation of systems such as automobiles, aircraft, spacecraft, medical devices, and robots, among others. The sensors can be used to measure one or more variables such as pressure, temperature, speed, acceleration, motion, proximity, and so forth. Sensor outputs may then be used as feedback in a closed-loop control operation to ensure that the system is operated at the desired conditions, that operational bounds are observed, and that system performance is optimized. Technological advances have enabled many more sensors to be manufactured on a microscopic scale using microelectromechanical systems (MEMS) technology. MEMS technology combines microelectronics with miniaturized mechanical systems such as valves, gears, and any other component or components on a semiconductor chip using nanotechnology. Such microsensors can operate at significantly higher speeds and with greater sensitivity as compared to macroscopic designs.
Many sensor applications require smaller size and low cost packaging to meet aggressive cost targets. In addition, sensor applications are increasingly calling for tighter accuracy specifications, especially with regard to error due to temperature variation. Two commonly used metrics to express the temperature variation of a sensor are temperature coefficient of offset (TCO) and temperature coefficient of sensitivity (TCS). The term “offset” refers to the output deviation from its nominal value at the non-excited state of the MEMS sensor. Thus, TCO is a measure of how much thermal stresses affect the offset of a semiconductor device, such as a MEMS sensor. A high TCO indicates correspondingly high thermally induced stress, or a MEMS device that is very sensitive to such stress. The packaging of MEMS sensor applications often uses materials with dissimilar coefficients of thermal expansion. Thus, an undesirable TCO often develops during manufacture or operation. Even in the case of a perfectly stress-isolated package, there may still be a substantial TCO due to other factors. The term “sensitivity” refers to the ratio of output change for a given change at the input. Thus, TCS is a measure of how the gain of the sensor changes as temperature varies. Sensors commonly need to be individually calibrated to meet the TCS and TCO specifications. The cost associated with individualized temperature trim can be a significant portion of the overall calibration/test cost. A temperature compensation circuit can be included with a sensor system that utilizes a plurality of thermistors which vary the magnitude of the excitation voltage across a transducer to compensate for the undesirable changes in sensitivity with temperature. For example, a pressure transducer is basically a bridge circuit and the thermistors are connected from each input terminal of the bridge to a power supply line. The thermistors change the excitation voltage level so that the output voltage across the terminals of the bridge remain constant for a given change in pressure even though the temperature changes. The thermistors are shunted with temperature stable elements such as resistors to tailor the compensation characteristic. The combination of resistors, thermistors, and transducer is adjusted by laser trimming through iterative operations over temperature to provide a device that provides accurate measurements over a range of temperatures. These adjustments can include sequential measurements over temperature and trimming. Individual resistive elements can also be trimmed to compensate for undesired temperature dependence of the transducer offset voltage. A complex procedure for trimming the resistors is also required. The gain of the circuit and the offset voltage of the transducer interact, requiring different pressures as well as temperatures for calibrating both gain and offset voltage.